A currently popular form of optical storage of information is the compact disk or CD. Digital information is stored in the form of low reflectivity marks or pits on an otherwise highly reflective background. In this format, the optical information is most often in the form of read only memory or ROM. Optical information is not usually recorded in real time but rather is produced by press molding. In a typical process, the optical recording substrate is first press molded with a master containing the digital information to be reproduced. The thus formed information is then overcoated with a metal, such as aluminum, to form a reflective layer and then with an optional protective layer. In those areas having the deformations or pits, the reflectivity is lower than in those areas not having the deformations.
Another CD storage element is recordable CD (CD-R). Recordable CD elements of this type are described in U.S. Pat. No. 4,940,618, European Patent Application 0,353,393 and Canadian Patent Application 2,005,520.
CD optical elements, including recordable CD elements, employing phthalocyanine dyes in the recording layer and gold as the reflector layer are particularly useful. The light stability of phthalocyanine dyes and the high reflectivity of gold are among the properties that render these materials of special value.
Although error correction codes are employed in recordable CD elements, it is important to prepare media substantially free of defects and to maintain the media in such a state through its lifetime so as to insure the integrity of the recorded data.
We have found that under certain conditions there is a tendency for recordable CD elements employing phthalocyanine dyes and a gold reflector layer to develop defects in the gold layer which range in size from 10 microns or less up to two hundred microns or more. Bits of information in CD-R media are of the order of one micron so that such defects can lead to a large loss of data and particularly when they are present at high areal densities. The defects are particularly prone to occur in recorded areas of the discs although they are observed in unrecorded media as well. High humidity is the environmental condition that most contributes to the defect growth and such is further aggravated at higher temperatures.